The present invention relates generally to the detection of surface flaws in manufactured articles and, more particularly, to the detection of surface flaws in cylindrically shaped articles such as can bodies.
Apparatus for detecting surface flaws in cylindrical objects are disclosed in U.S. Pat. Nos. 4,002,966; 4,029,958; 4,042,877; and 4,212,205, which are each hereby specifically incorporated by reference for all that is disclosed therein.
U.S. Pat. No. 4,212,205 of West et al. discloses a container defect detection apparatus which magnetically strikes containers causing them to ring at their natural frequency. A microphone senses the resonance; a bandpass filter filters out known frequencies generated by a known acceptable container. The energy of predetermined frequencies associated with defective containers is examined and compared with a threshold level to detect defects.
U.S. Pat. No. 4,002,966 of Hinds et al. discloses a method for detecting imperfections on the wall of cylindrical containers which includes generating a magnetic field having a predetermined value by passing an alternating current through an inductor means and then rotating a container body about its axis to pass all portions of a selected section of container body to be tested through the magnetic field while maintaining a fixed spacing between the tested portion of the container body and the inductor means and then comparing the rms value of the current producing the magnetic field with a reference value, and producing a signal when there is a predetermined difference between the two values.
U.S. Pat. No. 4,029,958 of Wright describes an apparatus for inspecting containers wherein a stream of cylindrical metal containers is fed into apparatus having a rotatable turret. The turret indexes each container past a driven wheel for imparting rotation thereto. A proximity detector disposed in close conjunction with the periphery of the rotating container detects any substantial runout of the periphery, indicating a dented container. A pair of elongated eddy current type transducers aligned axially along the sidewall of a juxtaposed rotating container are specifically described for use as a proximity detector.
U.S. Pat. No. 4,042,877 of Sieverin discloses a method of providing a substantially uniform gap between detection means and a substantially cylindrical object while it is being rotated in an inspection station formed by portions of the arcs of the circumferences of a pair of drive wheels mounted on a rotatably driven first shaft, and a pair of wheels mounted on a rotatably driven second shaft parallel to the first shaft.
U.S. Pat. No. 4,241,281 of Wu et al. discloses an apparatus for receiving used metallic containers and for dispensing a token or the like for the value of the container received. An object inserted into the apparatus passes through a control tube assembly which includes first and second inductive-type metal sensor units having coaxially aligned cylindrical passages and a third photoelectric-type sensor unit. The sensor units are arranged axially spaced apart and adjusted to the lengths of cans which are to be received. The first sensor unit provides a signal indicative of the passage of any metallic object; the second sensor unit provides a signal indicative of the passage of an aluminum object of predetermined minimum axial length; and the third sensor unit provides a signal indicative of the passage of any object of a predetermined minimum axial length. This patent is not directed to surface inspection of articles.
Thus, with exception of U.S. Pat. No. 4,212,205 which employs acoustic inspection techniques, the prior art relating to cylindrical article inspection apparatus teaches that a cylindrical article to be inspected for surface flaws is rotated within the sensing field of a sensor which is positioned alongside the article.